Centrifugal Pumps are significant energy consumers for many process industry. It is essential to operate pump at it best operating efficiency for minimum power consumption. With the advancement of technology large capacity centrifugal pumps are now available with above 90% efficiency.
While auditing large no of pumps it is found that only few pumps are being operated at its best efficiency point. In many cases the efficiency of centrifugal pumps goes down to below 60%.
Reason for lower efficiency is either pump is not being operated at its designed operating parameters or efficiency of pump gets deteriorated over time. For the pump which is not being operated at its designed operating curve. option of impeller resizing or pump replacement can be explored. Lets discuss on pump efficiency deterioration due to pump internal issue.
Detoriation of pump efficiency :-
Simplest method for checking pump efficiency deterioration is to put pump on shutoff head and check if it matches with designed shutoff head. If the shutoff head is lower than designed shutoff head, it means pump is having internal problem and exact problem needs to be identified and addressed for improving efficiency.
The reason for efficiency drop can be mechanical, volumetric or hydraulic, Mechanical losses are due to rotary components such as bearing, glands or seals. Volumetric losses are due to bypassing of discharge fluid to suction, in pump small amount of fluid is bypassed though wear ring. if the clearance in the wear rings increases the bypassing of fluid will increase and thus leasing to loss in pump efficiency. Hydraulic loss are caused by the frictional forces generated between the fluid and the walls of the hydraulic passage due to surface roughness,. Therefore friction should be minimized for operating pump at its highest efficiency. For problem related to increased surface roughness or minimize volumetric losses, coating can be a solution.
Loss of hydraulic efficiency :-
The reason for increased surface roughness can be due to below conditions:-
- Corrosion: Corrosion can be caused by oxidation of metal surfaces; it can be uniform or localized. Corrosion can also be due to bimetallic reaction. It happens when two metals of differing potentials are placed in solution and electrically connected together. Pitting is also a form of corrosion which may be casued due to internal structure of alloy. Deposits on the surface can also lead to corrosion.
- Erosion : with the cause of erosion, erosion is divided into two categories such as impingement erosion or entertainment erosion. Impingement erosion occurs due to higher fluid velocity while entertainment erosion occurs due other material such as silts or graves being carried along with high velocity fluid.
- Cavitation : it is the phenomenon where formation and collapsing of bubble occurs in the area of relative lower pressure around the impeller.
Conventionally problem of surface roughness is minimized with the use of right pump material selection. As this problem of surface roughness and loss of hydraulic efficiency can only be minimized but cannot be eliminated completely. Pumps are trends to lose its efficiency over a period of time. To put back pump in original efficiency condition coating can be a good option.
Coating Technology :-
To reduce surface roughness in the pump casing and impeller coating can be applied. Generally pump coatings are epoxy based. Pumps are removed and sand blasting is done in impeller and casing internals. Fillers are applied for filling the gaps and then coating is applied to improve corrosion-erosion resistance, hydrophobicity and hydraulic smoothness. Coating will reduce surface roughness and thus improve pump efficiency.
Surface roughness with coating can be achieved upto 0.09 µm while with polished SS material roughness can be upto 1.19 µm. This smooth surface reduces turbulence in the centrifugal pump. Lower roughness value of coating makes it beneficial even with the new pump.
Expected benefit with coating technology :-
As the benefit of pump coating depend on surface roughness. The expected benefit will vary with the actual corrosion-erosion in the pump internals. Coatings not only improve efficiency of pump but it also make is more corrosion resistant and improve pumps life expectancy. With new pump 2-3% efficiency gain can be observed easily with pump coating.
While with older pump efficiency improvement depends on the actual operating parameters and it can be much higher than 10%.Therefore It is recommended to use coating technology in pumps especially with old ones to gain on efficiency and life expectancy part.